High density electronic systems, such as the types developed for aerospace and military applications, are normally formed of standard size, two-sided electronic circuit modules typically having twenty-five (25) or more square inches of printed wiring board space on each side. These circuit modules require a combination of mechanical and thermal properties which enable the system to tolerate environmental stresses due to temperature excursions and/or vibration without electronic failures. Differences in material properties among the various components in prior circuit modules can lead to failures during normal operation.
In recent years, the military has been increasing its vibrational loading conditions and lowering the weight requirements for electronic circuit modules. Unfortunately, to withstand environmental stresses and to dissipate power, conventional surface mount technology printed wiring boards have been relying upon a heavy copper layered material which has, because of weight requirements, traditionally experienced structural problems under vibrational test and in the field.
The conventional surface mount technology of printed wiring boards (PWB) uses clad material for the core, such as copper-invar-copper (CIC) or copper-molybdenum-copper (CMC), as the constraining heat sink on high wattage circuit boards. Generally, current available CIC and CMC layered materials have relatively high densities which require the fabrication and use of relatively thin layers of copper in order to meet the weight requirements set by the military. Unfortunately, this thinning of the layers occurs at the expense of a lower resonant vibration frequency which ultimately reduces the stiffness of the board and lowers the cooling capacity.
Because of the reduced thickness of copper in prior devices, there is minimal dissipation of heat from the contacting electronic circuit modules. Further, the difference between the coefficient of thermal expansion (CTE) of the core and the electronic circuit modules is high, since the ratio of the electronic circuit modules volume to the core volume is high due to the thinning of the layers. This overall increase in the difference of the CTE of the components lowers the system's performance and requires the use of leaded ceramic chip carriers which ultimately decreases the available space on the surface of the circuit boards, and limits the performance and power.
To prevent damage of the core, CIC and CMC layers require corrosion protection of the invar and copper constituents. This has been accomplished in the past by plating the CIC or CMC layer with a nickel composition. This plating step, unfortunately, is costly and can be extremely difficult to perform. This is especially true on CMC layers where the nickel must be sintered onto the molybdenum at 850 degrees Centigrade to provide adequate nickel to molybdenum adhesion. However, this temperature of 850 degrees Centigrade is too high for copper and makes it difficult to find and operate in a processing window. Further, the processes that adequately nickel-plate the exposed metal surfaces and CMC layers are not generally compatible with an organic based circuit board because the organic circuit boards cannot tolerate the high temperature of 850 degrees Centigrade during the sintering process. Therefore, the finished surface mount technology PWB contain exposed metal surfaces of CIC and CMC which will create corrosion problems.
To offset the thin nature of prior CIC and CMC layers, the vibration resistance has been increased by incorporating structural stiffeners, such as ribs, to increase the structural rigidity of the module without dramatically increasing the core weight. However, the stiffeners require space on the module for attachment by screws which results in less space for the electronic device placement. The stiffeners further cause the following detrimental effects: increased module fabrication time, added weight associated with the stiffeners, added system costs associated with the fabrication of the stiffeners, and routing problems in design of PWB.
A need has therefore arisen for a lightweight surface mount composite core for printed circuit boards which has a high thermal conductivity associated with the composite core layers. Additionally, there is a need for a composite core which has a coefficient of thermal expansion (CTE) which is approximately the same as that of the electronic components in order to eliminate the CTE mismatch which results in cracking of the solder joints between the electronic components and the core. Finally, there is a need for a lightweight core for holding modules which has a high stiffness associated therewith in order to meet the high vibrational loading standards set by the military.